The memory CD8 T cell compartment can be activated by inflammatory cues even in the absence of a T cell receptor signal. This process is referred to as bystander-activation of memory T cells and has been observed following different infections. The contribution of these cells to host immunity was poorly understood until we recently reported that bystander-activated memory CD8 T cells play a crucial role in controlling early pathogen replication following infection. We demonstrated that bystander-activated memory CD8 T cells express granzyme B and can directly kill target cells in a TCR-independent, innate-like fashion. In stark contrast to their beneficial effect for host immunity following infection, our most recent data provide strong evidence that these bystander-activated T cells decrease antigen availability following vaccination by eliminating antigen- presenting cells. We propose to test the hypothesis that bystander-activated memory CD8 T cells play a fundamental and thus far unappreciated role in directly controlling the size of the cellular and humoral immune response following vaccination. We will determine which memory T cell subsets are capable of becoming bystander-activated, identify how bystander-activated T cells affect subsequent adaptive immune responses and define the mechanisms that lead to decreased antigen availability following vaccination. We will use a mouse model system to define the underlying mechanisms of target cell elimination and primary human T cells ex vivo to ensure direct relevance for human health. This proposal is highly significant because addressing the role of bystander-activated memory T cells in a vaccine context is one of the most significant and pressing cellular immunology research topics due to the inability to achieve effective immunity by vaccination in select populations at risk for and most susceptible to infections. This proposal is highly innovative because we identified NKG2D as a key molecule in this process and engineered a novel antagonist designed to enhance vaccine efficacy by inhibiting bystander-activated CD8 T cell function. Our proposed experiments clearly define the impact of bystander-activation on subsequent immune responses and to provide a new strategy for enhancing vaccine efficacy.